Thread: [Valgrind-users] No "Use of uninitialised value" warning

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Hi,

I'm using Valgrind 3.4.1 and I wonder why I don't get a warning for the 
std::swap() call in the attached example. I get one in printf(). Is this 
a bug?

Cheers,
André

André Wöbbeking wrote:
> Hi,
>
> I'm using Valgrind 3.4.1 and I wonder why I don't get a warning for the 
> std::swap() call in the attached example. I get one in printf(). Is this 
> a bug?
>
>
> Cheers,
> André
>   
> ------------------------------------------------------------------------

André,

Valgrind only reports when an unused variable is used for flow-control 
(or, hopefuly I/O).

Swapping doesn't trigger the alert. This is because, in C, structures 
can legitimately have
uninitialised areas due to nested unions or padding.

printf() uses flow-control to decide how many characters to print.

HTH,
Colin S. Miller

On Wed, May 13, 2009 at 2:43 AM, Colin Miller <col...@pi...> wrote:
> André Wöbbeking wrote:
>
> Valgrind only reports when an unused variable is used for flow-control
> (or, hopefuly I/O).
>
> Swapping doesn't trigger the alert. This is because, in C, structures
> can legitimately have
> uninitialised areas due to nested unions or padding.

There's more on this in the manual.  And this blog post discusses it
in even more detail:

http://blog.mozilla.com/nnethercote/2009/02/27/eliminating-undefined-values-with-valgrind-the-easy-way/

Nick

On Wednesday 13 May 2009, Nicholas Nethercote wrote:
> On Wed, May 13, 2009 at 2:43 AM, Colin Miller 
<col...@pi...> wrote:
> > André Wöbbeking wrote:
> >
> > Valgrind only reports when an unused variable is used for
> > flow-control (or, hopefuly I/O).
> >
> > Swapping doesn't trigger the alert. This is because, in C,
> > structures can legitimately have
> > uninitialised areas due to nested unions or padding.
>
> There's more on this in the manual.  And this blog post discusses it
> in even more detail:
>
> http://blog.mozilla.com/nnethercote/2009/02/27/eliminating-undefined-
>values-with-valgrind-the-easy-way/

Thanks.

Now back to my problem. The original code has double instead of int (I 
just tried both types to be sure and forgot to change back) and on MSVC 
std::swap() of uninitialised doubles triggers a floating point exception 
(if enabled) as MSVC uses the FPU.

I was really surprised that the code crashes on WIndows as it runs fine 
under valgrind on Linux.

Cheers,
André

> Now back to my problem. The original code has double instead of int (I
> just tried both types to be sure and forgot to change back) and on
MSVC
> std::swap() of uninitialised doubles triggers a floating point
> exception (if enabled) as MSVC uses the FPU.
> 
> I was really surprised that the code crashes on Windows as it runs
fine
> under valgrind on Linux.

This is a long-established quirk of Microsoft's compilers. They assume,
rather more deeply than most people, that floating point traps will be 
turned off. 

-- 
John Dallman
Parasolid Porting Engineer

Thread: [Valgrind-users] No "Use of uninitialised value" warning

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